Purpose: To retrospectively assess accuracy of magnetic resonance (MR) imaging after radiation therapy with concomitant chemotherapy for downsizing of the primary lesion to ypT0-2 tumor confined to rectal wall in locally advanced rectal cancer, with histopathologic findings as reference standard, and to evaluate additional value of volumetric analysis. Materials and Methods: The institutional review board approved the study and waived informed consent. Sixty-seven patients met criteria of the study. T2-weighted MR images obtained before and after radiation therapy with concomitant chemotherapy were assessed for tumor stage by expert abdominal radiologist, colorectal surgeon, and general radiologist. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated; tumor volume was measured (compared with Mann-Whitney U test). Findings were correlated with histopathologic findings. Results: Sixty-seven patients (38 men, 29 women; mean age, 63 years) who underwent radiation therapy with concomitant chemotherapy and surgery (all but one) were evaluated. The PPV for prediction of tumor confined to rectal wall (ypT0-2) was 91% (10 of 11), 86% (six of seven), and 88% (seven of eight) for expert abdominal radiologist, surgeon, and general radiologist, respectively. In 24 patients, sensitivity was 42% (10), 25% (six), and 29% (seven). ypT0-2 tumors had significantly smaller volumes than did ypT3-4 tumors before radiation therapy with concomitant chemotherapy (55 vs 92 cm(3), P = .038). Volume reduction rates were significantly higher in ypT0-2 than in ypT3-4 tumors (89% vs 61%, P < .001). If volume before radiation therapy with concomitant chemotherapy was 50 cm(3) or smaller and volume reduction rate was 75% or higher, excised tumor was always confined to rectal wall (ypT0-2). By using these criteria, 43% (six of 14) of cases with overstaging could have been predicted to be ypT0-2 tumors correctly. Conclusion: Downsizing to ypT0-2 tumors can be accurately predicted by combining morphologic tumor staging predictions with results from volumetric analyses. MR images obtained after radiation therapy with concomitant chemotherapy might be helpful in more individualized treatment planning. (c) RSNA, 2009.